1. Field of the Invention
The present invention relates to a power supply apparatus provided with a protection function for abnormal current of a circuit and a power supply apparatus control method.
2. Description of the Related Art
In the conventional art, such as the invention disclosed in Japanese Patent Laid-Open No. 07-288930, a technique is known in which, in a power supply circuit that supplies DC voltage power from a power supply circuit to a plurality of load circuits through separate power lines, a breaker element is provided on each power line. In this case, in a case in which a short occurs in the load circuits or in a DC-DC converter and an overcurrent flows through the power lines, a control circuit turns off the breaker element, enabling the current to be cut off without affecting other load circuits with a voltage fluctuation or the like.
Further, a conventional power supply circuit like that shown in FIG. 3 is also known. In FIG. 3, reference numeral 10 designates a power source such as a battery or the like, and 20 designates a DC-DC converter. DC voltage output from the DC-DC converter 20 is applied to load circuits 70 through 90 as DC power for operation. In addition, a current detection resistor 40 and a current limiting resistor 50 are connected in series between the power source 10 and the DC-DC converter 20.
Both ends of the current detection resistor 40 are connected to an abnormal current detection circuit 31 contained in a control block 30. In the control block 30 there is provided the abnormal current detection circuit 31 and a control circuit 32 to which is supplied the output of the abnormal current detection circuit 31. In addition, a breaker switch circuit 60 is provided in parallel with the current limiting resistor 50. The breaker switch circuit 60 is controlled by a control signal from the control circuit 32. The control signal from the control circuit 32 is supplied to the DC-DC converter 20, and is used to put the DC-DC converter 20 into a state of operation. The operating power of the control block 30 is supplied directly from the power source 10.
Assume that, in the conventional power supply protection circuit shown in FIG. 3 having the configuration described above, when the power source 10 is activated, that is, when the battery is installed, a short occurs either in the DC-DC converter 20 or in one of the load circuits 70 through 90. In that case, as a matter of course an abnormal current flows. In such a case in which an abnormal current flows when the power source 10 is activated, an abnormality is detected at the abnormal current detection circuit 31 connected across the current detection resistor 40. That abnormality is then reported to the control circuit 32, and the output from the control circuit 32 turns the breaker switch circuit 60 off. However, even when the breaker switch circuit 60 is off there is still the current limiting resistor 50, and some current continues to flow from the power source 10. As a result, in a case in which the current limiting resistor 50 heats unnecessarily, the power source 10 discharges unnecessarily, or the power source 10 is a secondary battery, there is a risk of deterioration in performance due to overcurrent.
In order to avoid the above-described disadvantage due to the current limiting resistor 50, the current limiting resistor 50 could be eliminated. If the current limiting resistor 50 is eliminated, then when the power source 10 is activated, when the DC-DC converter 20 or any of the load circuits connected downstream of the DC-DC converter 20 is shorted, the power source 10 terminal voltage drops, operating power for the control block 30 cannot be maintained, and a shorted state continues, and ultimately the role of a protection circuit cannot be fulfilled. Alternatively, it might be possible to increase the resistance of the current limiting resistor 50, but because the abnormal current detection circuit 31 requires a current of at least several hundred mA in order to detect an abnormal current, the current limiting resistor 50 resistance cannot be increased very much due to this problem of sensitivity.